1. Field of the Invention
The present invention relates to a method and to a device for monitoring and for dynamically balancing a storage battery pack.
2. Discussion of the Background
A battery is often called a pack, which term in fact covers a set of batteries. For example, in an electric vehicle, the "traction battery" contains several tens of batteries. These batteries exhibit all kinds of variations in characteristics, which very substantially affect the value of their maximum capacity, their charging/discharging dynamics and their ageing.
During a charge, some of the batteries reach their full charge well before others, without this being able to be detected by a charger, which measures only the overall voltage at the terminals of the pack and at a single temperature. Thus, therefore, the charging continuing, the batteries already fully charged become overcharged, ultimately with outgassing, and it may happen that, at the end of charging (period when the charging current is the smallest), some of the batteries are not fully charged.
During a discharge, the reverse problem occurs. The batteries not completely charged or those of different capacities (manufacturing variations) reach exhaustive discharge (hence degrade by irreversible sulphation of the plates in the case of lead batteries) before the batteries of larger capacity or those which had reached the full charge without a battery controller, which monitors only the overall voltage of the pack, being able to detect it. It may thus be seen that battery imbalance becomes worse with the number of charging/discharging cycles and the ageing of the batteries.
In order to alleviate this phenomenon partly, battery manufacturers are obliged to "match" the batteries of the same vehicle, something which is relatively expensive, only decreases the risk of imbalance and does not solve the problem when one battery of the pack is changed, during a maintenance check, or when the batteries age.
When the imbalance affects the characteristics of the pack (premature collapse of the voltage upon demand for heavy currents, abnormal heat-up or dangerous outgassing), the battery manufacturer's after-sales service must test the variation in voltage of each battery dynamically, for a given consumption. The reason for this is that the voltage at the terminals of a battery in open circuit is in no way characteristic of its state. This operation results in practically disconnecting each battery from the traction pack, which is lengthy, costly and laborious, and puts a brake on the development of electric vehicles.
Any extension in the lifetime of batteries represents a major advantage for the vehicle's owner. This is because the cost of a 6 V element of a 160 A.h traction battery is currently estimated to be approximately 1500.degree. F.